DOCS: added missing imports (#13736) (#13737)

- **Description:** Fixed missing imports in docs 
- **Issue:**
[#13736](https://github.com/langchain-ai/langchain/issues/13736)
- **Dependencies:** N/A

docs/docs/modules/model_io/prompts/prompt_templates/msg_prompt_templates.mdx

@@ -27,6 +27,8 @@ LangChain also provides `MessagesPlaceholder`, which gives you full control of w
 
 ```python
 from langchain.prompts import MessagesPlaceholder
+from langchain.prompts import HumanMessagePromptTemplate
+from langchain.prompts import ChatPromptTemplate
 
 human_prompt = "Summarize our conversation so far in {word_count} words."
 human_message_template = HumanMessagePromptTemplate.from_template(human_prompt)
@@ -36,6 +38,8 @@ chat_prompt = ChatPromptTemplate.from_messages([MessagesPlaceholder(variable_nam
 
 
 ```python
+from langchain.schema.messages import HumanMessage, AIMessage
+
 human_message = HumanMessage(content="What is the best way to learn programming?")
 ai_message = AIMessage(content="""\
 1. Choose a programming language: Decide on a programming language that you want to learn.